The government has been plunged into another row over its health reforms after an ambitious move by the Liberal Democrat leader, Nick Clegg, to cast himself as the saviour of the NHS was undermined by his Tory coalition partners.

In an attempt to avoid a damaging backlash at Lib Dem spring conference in Gateshead next week, the deputy prime minister joined forces with Lady Williams of Crosby in a letter promising MPs and peers a series of amendments to the health bill, to "rule out beyond doubt any threat of a US-style market in the NHS".

But the positioning was quickly undermined when the prime minister's spokesman said: "We have made it clear that we do not see any need for further significant changes to the bill."

The impression that the amendments, which Clegg had discussed with David Cameron and the Department of Health, were worth little was reinforced by health officials who suggested the changes were minor, and the Conservative chairman of the health select committee, Stephen Dorrell, who said they had all been previously agreed by ministers, or in the case of European competition law, were beyond the powers of Westminster.

Later, Lib Dems appeared to concede to the government's description of the changes as less like "amendments" and more like "reassurances".

In the letter, Clegg praises the Lib Dems for their part in more than 200 hours of scrutiny and more than 1,000 amendments to the health and social care bill, which he said "should guarantee the future of the NHS". But recognising continuing disquiet in his own party in the build up to the spring conference, he offered five more amendments to the controversial third part of the bill, which is supposed to introduce more competition to delivering services in the hope of cutting costs and allows hospitals to make up to 49% of their income from private patients, as long as profits are put back into the NHS.

Pressure on Clegg was ramped up when the party chairman, Tim Farron, said last week that the bill should have been dropped or "massively changed".

On Monday, hospital doctors looked set to become the latest medical body to formally oppose the reforms, after the Royal College of Physicians voted to ballot of members on dropping the bill.

The latest Lib Dem amendments include insistence that Monitor, the independent regulator of foundation trusts, will remain in that role unless specifically removed from it by the health secretary and parliament, and insulating the NHS from the "full force" of competition law.

"Once these final changes have been agreed, we believe conference can be reassured that it has finished the job it started last March [at the last spring conference] and the bill should be allowed to proceed," added the letter.

Andy Burnham, the Labour shadow health secretary, described the Lib Dems' tactics as an "unbelievable piece of self-justifying nonsense". He said: "Today's talk from the Lib Dems of minor amendments is too little, too late. They will only serve to further complicate a bill already described as a 'confused mess' by a former NHS chief executive."

Lib Dem MP Andrew George said members who oppose the bill would be less angry, but not happy. An expected emergency debate and vote on the bill at the conference in Gateshead would only be "less uncomfortable" for Clegg, he said.

Chris Nicholson, chief executive of the Clegg-supporting thinktank CentreForum, welcomed the letter but warned the bill was still confusing. "Nick and Shirley's letter treads a delicate path between emasculating the competition aspects of the bill, which would be a bad idea, and putting in enough safeguards to provide reassurance. I think it does that successfully.

"I think there are still problems with the bill with confused accountability and putting too much power in the hands of GP commissioning consortia. But this letter should address the main concern that Liberal Democrat activists have expressed."

Labour claimed the amendments were either copies or rewrites of their own proposed Lords amendments, but sometimes weaker.

Peers on Monday rejected a Labour amendment calling for GPs, who will have powers to commission health services for patients, to be penalised if they are found to have conflicts of interest, but accepted what Labour called a more "toothless" Lib Dem version setting up a register of interests.

Downing Street caused some irritation among Liberal Democrats by indicating that ministers would not agree to any significant changes to the bill. The prime minister's spokesman said: "We have made clear that we don't see any need for further significant changes to the bill. We had a listening exercise. We made significant changes to the bill. We think that the bill is the right one and the reforms are the right reforms."

Speaking a few hours later, after the publication of the Clegg-Williams letter, the No 10 spokesman declined to repeat his statement. Asked whether it was still the case the government would not be making significant changes to the bill, the spokesman said: "That is precisely what I said. The case this afternoon is that we accept that there are some people who require further clarity on some of these issues, particularly those issues relating to competition, want further reassurance and we are very happy to provide that."

A Department of Health source played down the significance of the changes, saying: "We are relaxed about it. This is not neutering or altering the purpose of the bill. We recognise that people feel strongly about it. That is fine."

Downing Street said the changes would provide clarity on competition, but that competition could still be increased under the amended bill. "This provides clarity on the role of competition. The level of competition is in the hands of clinical commissioning groups. Where they deem it appropriate to bring in other providers, and they see having other competing providers as a good thing to drive the quality of patient care, they will do that. That is still the case.

"The extent of competition is in the hands of GPs, clinical commissioning groups. They get to make choices about where providers should come from. They can do that in the best interests of their patients. That has always been the point of the reforms. That remains the case. Clearly we want to bring in new providers. If, by bringing in a new provider, you can improve care for patients then it would be strange not to do that. That is what the reforms do."

The Royal College of Physicians voted five to one to survey the college's 25,500 members so they can decide whether it should seek further changes to the bill or join other doctors' groups in demanding its withdrawal.

Almost all the speakers voiced serious concerns at the bill's potential impact on the NHS. Some 89% of the 189 RCP fellows present backed a motion which does not bind the RCP leadership and which said that the bill, if passed, "will damage the NHS and the health of the public in England".

Large majorities also approved other "indicative" motions urging the RCP to do more to oppose the bill. Doctors voted by 79% to 18% to call on the RCP to call publicly for the bill's withdrawal. Some 81% endorsed a call for it to join forces with other health bodies which ant the plan scrapped, such as the British Medical Association and royal colleges representing GPs, nurses, radiologists and others.

RCP president Sir Richard Thompson, who has been criticised by some members for not doing enough to oppose the bill despite criticising key elements of Lansley's plan at last week's Downing Street NHS "summit", said he would discuss with the RCP's ruling council how to act on the concerns raised.

Meanwhile, Lansley and David Cameron's belief that greater competition will improve the NHS receives a rare endorsement by a new report from Reform, the right of centre thinktank. Countries which have embraced competition in their healthcare systems have seen treatment improve and costs go down, according to a study of how new healthcare operators in the United States, India, Germany and elsewhere have managed to provide better quality care while also integrating services for patients.

"Parliament should pass the health bill, including the clauses on competition, not because the bill is perfect but because the NHS must move on," said Nick Seddon, Reform's deputy director. "If the NHS wants to be the best in the world, it should learn from the best in the world and the radical changes that other countries are implementing."